Apparatus and method for self-calibration of mass flow controller

ABSTRACT

A mass flow controller includes a mass flow sensor and a calibrator configured to perform a self-calibration on the mass flow controller. The calibrator generates flow measurements that are used as “standards” and flow measurements from the mass flow sensor are correlated to flow measurements from the calibrator.

BACKGROUND OF THE INVENTION

[0001] Capillary tube thermal mass flow sensors exploit the fact thatheat transfer to a fluid flowing in a laminar tube from the tube wallsis a function of mass flow rate of the fluid, the difference between thefluid temperature and the wall temperature, and the specific heat of thefluid. Mass flow controllers employ a variety of mass flow sensorconfigurations. For example, one type of construction involves astainless steel flow tube with one, and more typically two or more,resistive elements in thermally conductive contact with the sensor tube.The resistive elements are typically composed of a material having ahigh temperature coefficient of resistance. Each of the elements can actas a heater, a detector, or both. One or more of the elements isenergized with electrical current to supply heat to the fluid streamthrough the tube. If the heaters are supplied with constant current, therate of fluid mass flow through the tube can be derived from temperaturedifferences in the elements. Fluid mass flow rates can also be derivedby varying the current through the heaters to maintain a constanttemperature profile.

[0002] Such thermal mass flow sensors may be attached as a part of amass flow controller, with fluid from the controller's main channelfeeding the capillary tube (also referred to herein as the sensor tube).The portion of the main channel to which the inlet and outlet of thesensor tube are attached is often referred to as the “bypass” of theflow sensor. Many applications employ a plurality of mass flowcontrollers to regulate the supply of fluid through a supply line, and aplurality of the supply lines may be “tapped off” a main fluid supplyline. A sudden change in flow to one of the controller's may createpressure fluctuations at the inlet to one or more of the othercontrollers tapped off the main supply line. Such pressure fluctuationscreate differences between the flow rate at the inlet and outlet of anaffected mass flow controller. Because thermal mass flow sensors measureflow at the inlet of a mass flow controller, but outlet flow from thecontroller is the critical parameter for process control, suchinlet/outlet flow discrepancies can lead to significant process controlerrors.

[0003] In a semiconductor processing application, a process tool mayinclude a plurality of chambers with each chamber having one or moremass flow controllers controlling the flow of gas into the chamber. Eachof the mass flow controllers is typically re-calibrated every two weeks.The re-calibration process is described, for example, in U.S. Pat. No.6,332,348 B1, issued to Yelverton et al. Dec. 25, 2001, which is herebyincorporated by reference. In the course of such an “In Situ”calibration, conventional methods require a technician to connect a massflow meter in line with each of the mass flow controllers, flow gasthrough the mass flow meter and mass flow controller, compare the massflow controller reading to that of the mass flow meter and adjustcalibration constants, as necessary. Such painstaking operations canrequire a great deal of time and, due to labor costs and theunavailability of process tools, with which the mass flow controllersoperate, can be very costly.

[0004] A mass flow sensor that substantially eliminates sensitivity topressure variations would therefore be highly desirable. A convenientcalibration method and apparatus for mass flow controllers would also behighly desirable. More flexible access to a mass flow controller wouldalso be highly desirable. Apparatus and method for increasing thecontrol performance of a mass flow controller would also be highlydesirable.

SUMMARY OF THE INVENTION

[0005] In an illustrative embodiment, a mass flow controller inaccordance with the principles of the present invention includes thecombination of a thermal mass flow sensor and a pressure sensor toprovide a mass flow controller that is relatively insensitive tofluctuations in input pressure. The new controller is relativelyinexpensive, that is, it does not require a pair of expensive,precision, pressure sensors nor an all-stainless steel wetted surfacedifferential sensor. Nevertheless, the new controller is adapted tocontrol fluid flow over a broad range of fluid pressures. The new massflow controller includes a thermal mass flow sensor, a pressure sensor,and an electronic controller. The thermal mass flow sensor is configuredto measure the inlet flow of the controller. The pressure sensor sensesthe pressure within the volume in the channel between the flow sensor'sbypass and an outlet control valve, which volume will be referred toherein as the “dead volume.” The pressure sensor and thermal mass flowsensor respectively provide signals to the controller indicating themeasured inlet flow rate and the pressure within the dead volume. Atemperature sensor may be employed to sense the temperature of the fluidwithin the dead volume. In an illustrative embodiment, the temperaturesensor senses the temperature of the controller's wall, as anapproximation of the temperature of the fluid within the dead volume.The volume of the dead volume is determined, during manufacturing or acalibration process, for example, and may be stored or downloaded foruse by the electronic controller.

[0006] The controller employs the measured pressure within the deadvolume to compensate the measured inlet flow rate figure and to therebyproduce a compensated measure of the outlet flow rate as a function ofthe measured pressure and measure inlet flow rate. This compensatedmeasure of outlet flow rate may be used to operate a mass flowcontroller control valve. By reading the pressure sensor output over aperiod of time, the electronic controller determines the time rate ofchange of pressure within the dead volume. Given the dead volume, thetemperature of the fluid within the dead volume, and the input flow ratesensed by the thermal mass flow sensor, the electronic controllercomputes the fluid flow rate at the output of the mass flow controlleras a function of these variables. The electronic controller employs thiscomputed output fluid flow rate in a closed loop control system tocontrol the opening of the mass flow controller output control valve. Inan illustrative embodiment the pressure sensed by the pressure sensormay also be displayed, locally (that is, at the pressure sensor) and/orremotely (at a control panel or through a network interface, forexample).

[0007] In accordance with another aspect of the principles of thepresent invention, a variable-flow fluid source, a receptacle of knownvolume, and a pressure differentiator may be used to calibrate a massflow controller. The variable-flow fluid source supplies gas at varyingrates to the mass flow controller being calibrated and at proportionalrates to a receptacle of known volume. A pressure differentiatorcomputes the time derivative of gas flow into the receptacle of knownvolume and, from that, the actual flow into the receptacle. Given theactual flow, the proportionate flow into the mass flow controller may bedetermined and the flow signal from the mass flow controller correlatedto the actual flow. In an illustrative embodiment, a mass flowcontroller closes the outlet valve to form a receptacle of known volume(the dead volume). A pressure sensor located within the dead volumeproduces a signal that is representative of the pressure within the deadvolume. With the outlet valve closed, the flow into the dead volumedecreases exponentially while the pressure increases, until the pressurewithin the dead volume is equal to that at the inlet to the mass flowcontroller. The mass flow controller's electronic controller takes thetime derivative of the pressure at a plurality of times. Given the deadvolume/receptacle volume, the time derivative of the pressure within thedead volume, and the temperature of the gas, the controller computes theflow rate at those sample times. The electronic controller alsocorrelates the flow rates thus computed to the flow readings produced bythe mass flow controller's thermal mass flow sensor, thereby calibratingthe mass flow controller. This operation is self-contained, in that itdoesn't require the use of external mass flow meters or othercalibration devices. Various techniques and mechanisms may be employedto extend the period of time over which flow continues into the deadvolume, thereby permitting the computation of a greater number ofcorrelation, or calibration, points. For example, the outlet valve maybe fully opened before being shut at the beginning of a calibrationprocess or flow restrictors may be inserted at various locations withinthe gas flow path, for example.

[0008] In accordance with another aspect of the principles of thepresent invention, a mass flow controller includes an interface thatpermits an operator, such as a technician, to conduct diagnosticsthrough a network. Such diagnostics may be “active”, “passive”“on-line”, “off-line”, “manual”, or “automatic” or various combinationsof the above. By “active” diagnostics, we mean diagnostics that permitan operator to change drive signals in addition to, or instead of,monitoring signals. Enabling the use of drive signals permits atechnician to alter a test point setting, to thereby change currentthrough a resistor, for example. The technician may then monitor acorresponding signal, from a current sensor, for example. Or, atechnician may elect to alter the drive signal to a valve actuatordirectly, as opposed to setting a flow set-point and relying upon themass flow controller's electronic controller to adjust the valve drivesignal in the desired manner. Because such alterations present thepotential for creating flow control errors, access to such control maybe limited, through use of passwords and other security measures, forexample, at the network level. The term “passive” diagnostics refers todiagnostics that include monitoring functions, for example. The term“on-line” diagnostics is used to refer to diagnostics that are both realtime and operating concurrently with the mass flow controller's processcontrol operations. The term “off-line” diagnostics refers todiagnostics that, although they may be real time, are not operatingduring a mass flow controller's process control operations. The term“automatic” diagnostics refers to diagnostics include a plurality ofdiagnostic steps, each of which may be active or passive. The term“manual” diagnostics refers to diagnostics that are responsive on a stepby step basis, to an operator's input.

[0009] A mass flow controller in accordance with another aspect of theprinciples of the present invention includes a web server that permitsan operator, such as a technician, to interact with the mass flowcontroller from a web-enabled device, such as a workstation, laptopcomputer, or personal digital assistant, for example, over aninterworking network, such as the Internet. The mass flow controller webserver may include web pages that provide manufacturer's part number,specification, installation location, and performance information, forexample. Additionally, diagnostics may be conducted from a web-enableddevice over an interworking network.

[0010] In accordance with yet another aspect of the principles of thepresent invention, a mass flow controller may include a pressure displaythat displays the pressure within the mass flow controller. The displaymay be local, that is, directly in contact with or supported by the massflow controller, or the display may be remote, at a gas box controlpanel, for example. In an illustrative embodiment, a pressure sensor ispositioned to measure the pressure within a mass flow controller's deadvolume and that is the pressure that is displayed.

[0011] A dual-processor mass flow controller in accordance with thestill another aspect of the principles of the present invention includesa deterministic processor that performs the mass flow controllers'control duties and a non-deterministic processor that handles such tasksas providing a user interface. In an illustrative embodiment, thedeterministic processor is a digital signal processor (DSP).

[0012] In accordance with yet another aspect of the principles of thepresent invention, a plurality of executable code sets may be uploadedby a mass flow controller's electronic controller. In an illustrativeembodiment, a dual processor mass flow controller's non-deterministicprocessor uploads a plurality of executable codes sets for thedeterministic processor and selects among the code sets for thedeterministic processor to execute. Such selection by thenon-deterministic processor may enable a form of customization.

[0013] These and other advantages of the present disclosure will becomemore apparent to those of ordinary skill in the art after having readthe following detailed descriptions of the preferred embodiments, whichare illustrated in the attached drawing figures. For convenience ofillustration, elements within the Figures may not be drawn to scale.

BRIEF DESCRIPTION OF DRAWINGS

[0014]FIG. 1 is a block diagram of a system that includes a mass flowsensor in accordance with the principles of the present invention;

[0015]FIG. 2 is a sectional view of a mass flow controller that employsa mass flow sensor in accordance with the principles of the presentinvention;

[0016]FIG. 3 is a sectional view of an illustrative thermal mass flowsensor as used in conjunction with a pressure sensor to produce acompensated indication of mass flow through a mass flow controller;

[0017]FIG. 4 is a block diagram of the control electronics employed byan illustrative embodiment of a mass flow sensor in accordance with theprinciples of the present invention;

[0018]FIG. 5 is a flow chart of the process of compensating a thermalmass flow sensor signal in accordance with the principles of the presentinvention;

[0019]FIG. 6 is a conceptual block diagram of a web-enabled mass flowcontroller in accordance with the principles of the present invention;

[0020]FIG. 7 is a conceptual block diagram of calibrator such as may beemployed with a mass flow controller in accordance with the principlesof the present invention;

[0021]FIG. 8 is a block diagram of a self-calibrating mass flowcontroller in accordance with the principles of the present invention;

[0022]FIG. 9 is a graphical representation of flow and pressure curvescorresponding to the process of calibrating a mass flow controller inaccordance with the principles of the present invention;

[0023]FIG. 10 is a conceptual block diagram of a dual processorconfiguration such as may be used in a mass flow controller inaccordance with the principles of the present invention;

[0024]FIG. 11 is a flow chart of the general operation of a mass flowcontroller's non-deterministic processor in accordance with theprinciples of the present invention;

[0025]FIGS. 12A and 12B are flow charts of the general operation of amass flow controller's deterministic processor in accordance with theprinciples of the present invention; and

[0026]FIGS. 13A through 13E are screen shots of web pages such as may beemployed by a web server embedded within a mass flow controller inaccordance with the principles of the present invention.

DETAILED DESCRIPTION OF DISCLOSURE

[0027] A mass flow sensor in accordance with one aspect of theprinciples of the present invention employs a thermal mass flow sensorto sense and provide a measure of the flow of fluid into an inlet of afluid flow device, such as a mass flow controller. In an illustrativeembodiment, the mass flow sensor uses a pressure sensor to compensatethe inlet flow measure provided by the thermal mass flow sensor tothereby provide an indicator that more accurately reflects the fluidflow at the outlet of the associated mass flow controller. A system 100that benefits from and includes the use of a mass flow sensor inaccordance with the principles of the present invention is shown in theillustrative block diagram of FIG. 1.

[0028] A plurality of mass flow controllers MFC1, MFC2, . . . MFCnreceive gas from main gas supply lines 102,103. The mass flowcontrollers, MFC1, MFC2, . . . MFCn are respectively connected throughinlet supply lines 104, 106, . . . 109 to a main gas supply line 102,103and through respective outlet supply lines 110, 112, . . . 115 tochambers C1, C2, . . . Cn. In this illustrative embodiment, the term“chamber” is used in a broad sense, and each of the chambers may be usedfor any of a variety of applications, including, but not limited to,reactions involved in the production of semiconductor components.Generally, users of the chambers are interested in knowing andcontrolling the amount of each gas supplied to each of the chambers C1,C2 . . . Cn. Each chamber C1, C2, . . . Cn may also include one or moreadditional inlet lines for the supply of another type of gas. Outflowfrom the chambers may be routed through lines (not shown) for recyclingor disposal.

[0029] The mass flow controllers, MFC1, MFC2, . . . MFCn, includerespective mass flow sensors MFS1, MFS2, . . . MFSn, electroniccontrollers EC1, EC2 . . . ECn and outlet control valves OCV1, OCV2, . .. OCVn. At least one of the mass flow sensors is, and, for ease ofdescription, assume all are, compensated mass flow sensors in accordancewith the principles of the present invention. Each mass flow sensorsenses the mass of gas flowing into the mass flow controller andprovides a signal indicative of the sensed value to a correspondingelectronic controller. The electronic controller compares the indicationof mass flow as indicated by the sensed value provided by the mass flowsensor to a set point and operates the outlet control valve to minimizeany difference between the set point and the sensed value provided bythe mass flow sensor. Typically, the set point may be entered manually,at the mass flow controller, or downloaded to the mass flow controller.The setpoint may be adjusted, as warranted, through the intervention ofa human operator or automatic control system. Each of the inlet supplylines 104, 106, . . . 109 may be of a different gauge, and/or may handleany of a variety of flow rates into the mass flow controller. Inaccordance with one aspect of the principles of the present invention, asingle electronic controller, such as electronic controller EC1, may belinked to and operate a plurality of mass flow sensor/outlet controlvalve combinations. That is, for example, any number of the illustratedelectronic controllers EC2 through ECn may be eliminated, with thecorresponding mass flow sensors and outlet control valves linked to theelectronic controller EC1 for operation.

[0030] An abrupt change of flow rate, due to a change in set point forexample, into any of the mass flow controllers may be reflected as anabrupt pressure change at the inlet of one or more of the other massflow controllers. This unwanted side effect may be more pronounced in arelatively low flow rate mass flow controller if the abrupt changeoccurs in a high flow-rate mass flow controller. Because the mass flowsensors in this illustrative embodiment are thermal mass flow sensorspositioned to sense flow in the mass flow controller at the inlet to themass flow controller, the mass flow sensed by the thermal mass flowsensor may not accurately reflect the flow at the outlet of thecontroller. In order to compensate for this discrepancy, a mass flowsensor in accordance with one aspect of the principles of the presentinvention includes a pressure sensor positioned to provide an indicationof the pressure within the volume between the inlet and outlet of themass flow controller. In an illustrative embodiment, the pressure sensoris located in the “dead volume” between the thermal mass flow sensor'sbypass and the outlet control valve. An electronic controller employsthe indication of pressure provided by the pressure sensor to compensatethe measure of mass flow provided by the thermal mass flow sensor. Theresultant, a compensated mass flow indication, more accurately reflectsthe flow at the outlet of the mass flow controller and, consequently,this indication may be employed to advantage by a mass flow controllerin the operation of its outlet control valve. A display may be includedto display the sensed pressure. The display may be local, attached to orsupported by the mass flow controller, or it may be remote, at a gas boxcontrol panel, for example, connected to the mass flow controllerthrough a data link.

[0031] In a semiconductor processing application, a process tool mayinclude a plurality of chambers with each chamber having a plurality ofmass flow controllers respectively controlling the flow of constituentgases into the chamber. Each of the mass flow controllers is typicallyre-calibrated every two weeks. The re-calibration process is described,for example, in U.S. Pat. No. 6,332,348 B1, issued to Yelverton et al.Dec. 25, 2001, which is hereby incorporated by reference. In the courseof such an “In Situ” calibration, conventional methods require atechnician to connect a mass flow meter in line with each of the massflow controllers, flow gas through the mass flow meter and mass flowcontroller, compare the mass flow controller reading to that of the massflow meter and adjust calibration constants, as necessary. Suchpainstaking operations can require a great deal of time and, due tolabor costs and the unavailability of process tools with which the massflow controllers operate, can be very costly. In an illustrativeembodiment described in greater detail in the discussion related to FIG.7, a mass flow controller in accordance with the principles of thepresent invention includes a self-calibrating mechanism thatsubstantially eliminates such tedious and costly chores.

[0032] The sectional view of FIG. 2 provides an illustration of a massflow controller 200 that employs a mass flow sensor 202 in accordancewith one aspect of the principles of the present invention. The massflow sensor 202 includes a thermal mass flow sensor 204, a pressuresensor 206, a temperature sensor 208 and an electronic controller 210. Abypass 212 establishes a pressure drop across the capillary tube of thethermal mass flow sensor 204, as will be described in greater detail inthe discussion related to FIG. 3. In operation, a fluid that isintroduced to the mass flow controller 200 through the inlet 214proceeds through the channel 216 containing the bypass 212. A relativelysmall amount of the fluid is diverted through the thermal mass flowsensor 204 and re-enters the channel 216 downstream of the bypass 212.The electronic controller 210 provides a signal to the control valveactuator 218 to thereby operate the outlet control valve 220 in a waythat provides a controlled mass flow of fluid to the outlet 222. Thepressure sensor 206 senses the pressure within the volume within thechannel 216 between the bypass 212 and the outlet control valve 220,referred to herein as the “dead volume.” As will be described in greaterdetail in the discussion related to FIG. 5, the electronic controller210 employs the pressure sensed within the dead volume by the sensor 206to compensate the inlet flow rate sensed by the thermal flow sensor 204.This compensated inlet flow rate figure more closely reflects the outletflow rate, which is the ultimate target of control. In particular, amass flow sensor in accordance with one aspect of the principles of thepresent invention is a combination sensor that employs the time rate ofchange of pressure within a known volume to provide a precise measure ofmass flow during pressure transients and a thermal mass flow sensor thatmay be “corrected” using the pressure-derived mass flow measurement.Both the thermally-sensed and pressure-derived mass flow measurementsare available for processing. The temperature sensor 208 senses thetemperature of the fluid within the dead volume. In an illustrativeembodiment, the temperature sensor 208 senses the temperature of a wallof the controller, as an approximation of the temperature of the fluidwithin the dead volume.

[0033] The volume of the dead volume is determined, during manufacturingor a calibration process, for example, and may be stored or downloadedfor use by the electronic controller 210. By taking sequential readingsfrom the pressure sensor output and operating on that data, theelectronic controller 210 determines the time rate of change of pressurewithin the dead volume. Given the dead volume, the temperature of thefluid within the dead volume, the input flow rate sensed by the thermalmass flow sensor 204, and the time rate of change of pressure within thedead volume, the electronic controller 210 approximates the fluid flowrate at the output of the mass flow controller. As previously noted,this approximation may also be viewed as compensating the mass flow ratefigure produced by the thermal mass flow sensor. The electroniccontroller 210 employs this computed output fluid flow rate in a closedloop control system to control the opening of the mass flow controlleroutlet valve 220. In an illustrative embodiment the value of thepressure sensed by the pressure sensor may also be displayed, locally(that is, at the pressure sensor) and/or remotely (at a control panel orthrough a network interface, for example). In a self-calibrating processdescribed hereinafter in the discussion related to FIG. 7, theelectronic controller 210 may take the time derivative of the pressuresignal when the flow rate varies in the mass flow controller and therebyderive the actual flow rate into the mass flow controller. The actualflow rate may then be used to calibrate the mass flow controller.

[0034] The sectional view of FIG. 3 provides a more detailed view of athermal mass flow sensor, such as may be employed in conjunction with apressure sensor to produce a compensated mass flow indication that is,in a digital implementation, a multi-bit digital value. The multi-bitdigital value provides a closer approximation to the actual mass flow atthe outlet of a mass flow controller than an uncompensated mass flowsensor would, particularly during pressure transients on the mass flowcontroller inlet lines. The thermal mass flow sensor includes laminarflow element 212, which rests within the channel 216 and provides apressure drop across the bypass channel for the thermal mass flow sensor204 and drives a portion of the gas through the sensor capillary tube320 of the thermal mass flow sensor 204. The mass flow sensor includescircuitry that senses the rate of flow of gas through the controller 100and controls operation of the valve 320 accordingly. The mass flowsensor assembly 204 is attached to a wall 322 of the mass flowcontroller that forms a boundary of the bypass channel 216. Input 324and output 326 apertures in the wall 322 provide access to the mass flowsensor assembly 204 for a gas travelling through the mass flowcontroller and it is the portion of this passageway between the inputand output that typically defines the bypass channel. In thisillustrative embodiment the mass flow sensor assembly 204 includes abaseplate 328 for attachment to the wall 322. The baseplate 328 may deattached to the wall and to the remainder of the sensor assembly usingthreaded hole and mating bolt combinations, for example. Input 330 andoutput 332 legs of the sensor tube 320 extend through respective input334 and output 336 apertures of the baseplate 328 and, through apertures324 and 326, the mass flow controller wall 322.

[0035] The mass flow sensor assembly preferably includes top 338 andbottom 340 sections that, when joined, form a thermal clamp 341 thatholds both ends of the sensor tube active area (that is, the areadefined by the extremes of resistive elements in thermal contact withthe sensor tube) at substantially the same temperature. The thermalclamp also forms a chamber 342 around the active area of the sensor tube320. That is, the segment of the mass flow sensor tube within thechamber 342 is in thermal communication with two or more resistiveelements 344, 346, each of which may act as a heater, a detector, orboth. One or more of the elements is energized with electrical currentto supply heat to the fluid as it streams through the tube 320. Thethermal clamp 341, which is typically fabricated from a materialcharacterized by a high thermal conductivity relative to the thermalconductivity of the sensor tube, makes good thermally conductive contactwith the portion of the sensor tube just downstream from the resistiveelement 344 and with the portion of the sensor tube just upstream fromthe resistive element 346. The thermal clamp thereby encloses andprotects the resistive element 344 and 346 and the sensor tube 320.Additionally, the thermal clamp 341 thermally “anchors” those portionsof the sensor tube with which it makes contact at, or near, the ambienttemperature. In order to eliminate even minute errors due to temperaturedifferentials, the sensor tube may be moved within the thermal clamp toinsure that any difference between the resistance of the two coils isdue to fluid flow through the sensor tube; not to temperature gradientsimposed upon the coils from the environment.

[0036] In this illustrative embodiment, each of the resistive elements344 and 346 includes a thermally sensitive resistive conductor that iswound around a respective portion of the sensor tube 320. Each of theresistive elements extends along respective portions of the sensor tube320 along an axis AX1 defined by the operational segment of the sensortube 320. Downstream resistive element 344 is disposed downstream of theresistive element 346. The elements abut one another or are separated bya small gap for manufacturing convenience and are preferablyelectrically connected at the center of the tube. Each resistive elementprovides an electrical resistance that varies as a function of itstemperature. The temperature of each resistive element varies as afunction of the electrical current flowing through its resistiveconductor and the mass flow rate within the sensor tube. In this way,each of the resistive elements operates as both a heater and a sensor.That is, the element acts as a heater that generates heat as a functionof the current through the element and, at the same time, the elementacts as a sensor, allowing the temperature of the element to be measuredas a function of its electrical resistance. The mass flow sensor 302 mayemploy any of a variety of electronic circuits, typically in aWheatstone bridge arrangement, to apply energy to the resistive elements346 and 344, to measure the temperature dependent resistance changes inthe element and, thereby, the mass flow rate of fluid passing throughthe tube 320. Circuits employed for this purpose are disclosed, forexample, in U.S. Pat. No. 5,461,913, issued to Hinkle et al and U.S.Pat. No. 5,410,912 issued to Suzuki, both of which are herebyincorporated by reference in their entirety.

[0037] In operation, fluid flows from the inlet 214 to the outlet 222and a portion of the fluid flows through the restrictive laminar flowelement 212. The remaining and proportional amount of fluid flowsthrough the sensor tube 320. The circuit (not shown here) causes anelectrical current to flow through the resistive elements 344 and 346 sothat the resistive elements 344 and 346 generate and apply heat to thesensor tube 320 and, thereby, to the fluid flowing through the sensortube 320. Because the upstream resistive element 346 transfers heat tothe fluid before the fluid reaches the portion of the sensor tube 320enclosed by the downstream resistive element 344, the fluid conductsmore heat away from the upstream resistive element 346 than it does fromthe downstream resistive element 344. The difference in the amount ofheat conducted away from the two resistive elements is proportional tothe mass flow rate of fluid within the sensor tube and, by extension,the total mass flow rate through the mass flow rate controller from theinput port through the output port. The circuit measures this differenceby sensing the respective electrical resistances and generates an outputsignal that is representative of the mass flow rate through the sensortube.

[0038] The conceptual block diagram of FIG. 4 illustrates thearchitecture of an electronic controller 400 such as may be used in amass flow sensor in accordance with the principles of the presentinvention. In this illustrative embodiment, the controller 400 includessensor 402 and actuator 404 interfaces. Among the sensor interfaces 402,a flow sensor interface 408 operates in conjunction with a mass flowsensor to produce a digital representation of the rate of mass flow intoan associated mass flow controller. The controller 400 may includevarious other sensor interfaces, such as a pressure sensor interface 410or a temperature sensor interface 412. One or more actuator drivers 412are employed by the controller 400 to control, for example, the openingof an associated mass flow controller's output control valve. Theactuator may be any type of actuator, such as, for example, acurrent-driven solenoid or a voltage-driven piezo-electric actuator.

[0039] The controller 400 operates in conjunction with a mass flowcontroller to produce a digital representation of the rate of mass flowinto an associated mass flow controller. A thermal mass flow controller,such as described in the discussion related to FIG. 3, may be employedto produce the mass flow measurement. The controller 400 may employ apressure sensor interface 410 to monitor the pressure of fluid within anassociated mass flow controller. In an illustrative embodiment, apressure sensor, such as the pressure sensor 206 of FIG. 2, provides ameasure of the pressure within the mass flow controller. Morespecifically, in this illustrative embodiment, the sensor measures thepressure within dead volume of the mass flow controller. In anillustrative embodiment, the mass flow controller pressure thus measuredmay be displayed, at the pressure sensor 206 or at the controllerhousing, for example, or some other location.

[0040] The controller 400 may convert the pressure measurement todigital form and employ it in analysis or other functions. For example,if the mass flow controller employs a thermal mass flow sensor, thecontroller 400 may use the mass flow controller pressure measurement tocompensate for inlet pressure transients. Although a temperature sensorinterface may be used to obtain a temperature reading from a temperaturesensor attached, for example, to the wall of a mass flow controller, aseparate temperature sensor may not be required for each mass flowcontroller. For example, mass flow controllers are often employed, asdescribed in greater detail in the discussion related to FIG. 1, inconjunction with a semiconductor processing tool that includes a numberof mass flow controllers and other devices that are all linked to acontroller, such as a workstation. The processing tool is operatedwithin a carefully controlled environment that features a relativelystable temperature. Because the temperature of the fluid within the massflow controller is very nearly equal to that of the wall of theenclosure and the wall of the enclosure is very nearly the temperatureof the room within which the tool is housed, a temperature measurementfrom, for example, the workstation that controls the tool, may provide asufficiently accurate estimate of the gas temperature within the massflow controller. Consequently, in addition to, or instead of, employinga separate temperature sensor on each mass flow controller, thetemperature may be obtained from another sensor within the sameenvironment as the mass flow controller: one located at a workstation,for example.

[0041] The controller 400 includes a local user interface 416 that maybe used with one or more input devices, such as a keypad, keyboard,mouse, trackball, joy stick, buttons, touch screens, dual inlinepackaged (DIP) or thumb-wheel switches, for example, to accept inputfrom users, such as technicians who operate a mass flow controller. Thelocal user interface 414 may also include one or more outputs suitablefor driving one or more devices, such as a display, which may be anindicator light, a character, alphanumeric, or graphic display, or anaudio output device used to communicate information from a mass flowcontroller to a user, for example. A communications interface 416permits a mass flow controller to communicate with one or more otherinstruments, and/or with a local controller, such as a workstation thatcontrols a tool that employs a plurality of mass flow controllers and/orother devices in the production of integrated circuits, for example.

[0042] In this illustrative example, the communications interface 414includes a DeviceNet interface. DeviceNet is known and discussed, forexample, in U.S. Pat. No. 6,343,617 B1 issued to Tinsley et al. Feb. 5,2002, which is hereby incorporated by reference. The controller 400 alsoincludes storage 418 in the form, for example, of electrically erasableprogrammable read only memory (EEPROM) that may be used to storecalibration data, mass flow controller identification, or code foroperating the mass flow controller, for example. Various other forms ofstorage, such as random access memory (RAM), may be employed. Thestorage can take many forms, and, for example, may be distributed, withportions physically located on a controller “chip” (integrated circuit)and other portions located off-chip. The controller 400 employs a dataprocessor 420, which might take the form of an arithmetic logic unit(ALU) in a general purpose microprocessor, for example, to reduce data.For example, the data processor 420 may average readings received at thesensor inputs, determine the number of times a sensor reading hasexceeded one or more threshold values, record the time a sensor readingremains beyond a threshold value, or perform other forms of datalogging.

[0043] Pressure transients on the inlet supply line to a mass flowcontroller that employs a thermal mass flow sensor may create erroneousmass flow readings. Erroneous mass flow readings may lead, in turn, toimproper control of a mass flow controller's outlet valve, which coulddamage or destroy articles being processed with gasses under control ofthe mass flow controller. The digital representation of mass flow maytake the form of one or more data values and is subject to fluctuationsdue to pressure transients on the inlet line of the mass flow sensor. Inan illustrative embodiment, the controller 400 employs data obtained atthe pressure sensor interface 410 to compensate for fluctuations inducedin a thermal mass flow sensor by pressure transients on the mass flowsensor inlet line. In this illustrative embodiment, the controller 400obtains temperature information through a temperature interface 412. Thecontroller 400 employs the temperature, pressure, and mass flow readingsobtained from the respective interfaces, to produce a compensated massflow reading that more closely reflects the mass flow at the outlet ofthe mass flow sensor than a reading from the thermal mass flow sensoralone provides. The controller 400 also provides control to sensors, asnecessary, through flow sensor interface, pressure sensor interface, andtemperature interfaces, 408, 410, and 412, respectively.

[0044] The controller 400 also includes a valve actuator interface 404,which the controller 400 employs to control the position of a valve,such as the valve 220 of FIG. 2, to thereby control the rate of fluidflow through a mass flow controller, such as the mass flow controller200, in a closed-loop control process. The valve actuator may be asolenoid-driven actuator or peizo-electric actuator, for example. Thecontroller 400 must be capable of operating with sufficient speed toread the various sensor outputs, compensate as necessary, and adjust themass flow controller outlet valve to produce a predetermined flow rate.The flow rate is predetermined in the sense that it is “desired” in somesense. It is not predetermined in the sense that it must be a staticsetting. That is, the predetermined flow rate may be set by an operatorusing a mechanical means, such as a dial setting, or may be downloadedfrom another controller, such as a workstation, for example, and may beupdated.

[0045] In an illustrative embodiment, the controller 400 employsreadings from the pressure interface 410 to compensate flow measurementsobtained at the mass flow interface 408 from a thermal mass flow sensorthat senses mass flow at the inlet to a mass flow controller. Thecompensated flow measurement more accurately depict the flow at theoutlet of the mass flow controller. This outlet flow is the flow beingdirectly controlled by the mass flow controller and typically is theflow of interest to end users. Employing a pressure-compensated flowmeasurement in accordance with the principles of the present inventionimproves the accuracy of a mass flow sensor's outlet flow reading andthereby permits a mass flow controller to more accurately control theflow of fluids. That is, at equilibrium, mass flow at a mass flowcontroller's inlet is equal to the mass flow at the outlet of the massflow controller, but during inlet or outlet pressure transients, theflow rates differ, sometimes significantly. As a result, a mass flowcontroller that provides closed loop control using its inlet flow tocontrol its outlet flow may commit substantial control errors.

[0046] The steady state mass flow in the capillary tube of a thermalmass flow sensor such as described in the discussion related to FIG. 3is generally described by the following equation: $\begin{matrix}{Q_{c} = {\frac{d_{c}^{2}}{32\quad \mu}\frac{P_{i}}{P_{R}}\left( \frac{P_{i} - P}{L_{c}} \right)}} & (1)\end{matrix}$

[0047] where:

[0048] dc=capillary tube inside diameter

[0049] Lc=capillary tube length

[0050] ρi=the density of the gas at the inlet

[0051] ρR=the density of the gas at standard temperature and pressure

[0052] μ=the gas viscosity

[0053] Pi=the pressure at the inlet of the mass flow controller

[0054] Po=The pressure at the outlet of the mass flow controller

[0055] P=the pressure in the dead volume of the mass flow controller

[0056] The total flow through the mass flow controller is related tothat through the capillary tube through a split ratio:

α≡QBP/Qc

[0057] where QBP is the flow through the bypass and Qc is the flowthrough the capillary tube. The total flow at the mass flow controllerinlet is:

Qi=QBP+Qc=(1+α)Qc

[0058] If flow remains laminar in both the bypass and capillary, thesplit ratio will remain constant. When the inlet pressure varies withtime, the nature of the inlet pressure transient and the pressurizationof the dead volume govern the flow at the inlet. Assuming that allthermodynamic events within the dead volume occur at a constanttemperature that is equal to the temperature of the enclosure that formsa partial receptacle around the dead volume, the mass conservationwithin the dead volume may be described by: $\begin{matrix}{Q_{o} = {Q_{i} - {\frac{T_{R}V}{T_{w}P_{R}}\frac{P}{t}}}} & (2)\end{matrix}$

[0059] Where:

[0060] PR=pressure at standard temperature and pressure (760 Torr.)

[0061] TR=temperature at standard temperature and pressure (273 K)

[0062] Tw=wall temperature (temperature of the wall of the mass flowcontroller)

[0063] V=volume of the dead volume

[0064] Qi=inlet flow to the mass flow controller

[0065] Q0=outlet flow from the mass flow controller

[0066] A mass flow sensor in accordance with the principles of thepresent invention employs the relationship of equation (2) to compensatea thermal mass flow sensor's mass flow signal and to therebysubstantially reduce errors in mass flow readings during pressuretransients.

[0067] The flow chart of FIG. 5 depicts the process of compensating athermal mass flow sensor reading in accordance with the principles ofthe present invention. The process begins in step 500 and proceeds fromthere to step 502 where a mass flow sensor's controller, such as thecontroller 400 of FIG. 4, obtains a mass flow reading. This reading maybe obtained from a thermal mass flow sensor through a flow interface,such as interface 408 of FIG. 4, for example. This flow measurementreflects the rate of mass flow at the inlet of a mass flow controllerand, as previously described, may not adequately represent the mass flowrate at the outlet of the mass flow controller. The mass flow rate atthe outlet of a mass flow controller is generally the rate of interestfor use in control applications. Consequently, a mass flow controller inaccordance with the principles of the present invention compensates forthe inaccuracy inherent in assuming that the inlet flow rate to a massflow controller is equal to the outlet flow rate from the mass flowcontroller. From step 502 the process proceeds to step 504 where thesensor controller obtains the temperature. The temperature could beobtained through a temperature interface such as interface 412 of FIG.4, or it may be downloaded to the compensated mass flow sensor. Thecompensation process may safely assume that the gas temperature is equalthe temperature of the enclosure of the mass flow controller.Additionally, in most applications, the temperature will remainrelatively stable over a long period of time, so that a storedtemperature value may be employed, with updates as necessary.

[0068] After obtaining the gas temperature in step 504 the processproceeds to step 506 where the sensor controller obtains the volume ofthe dead volume. This value may have been stored during manufacturing,for example. From step 506 the process proceeds to step 508 where thepressure within the dead volume is obtained over a period of time. Thenumber of measurements and the time over which the measurements are madedepend upon the speed and duration of transients at the inlet of themass flow controller. In step 510 the processor employs the pressuremeasurements made in step 508 to compute the time rate of change ofpressure within the dead volume. After computing the time rate of changeof pressure within the dead volume, the process proceeds to step 512where a compensated outlet flow value is computed according to equation(2). Simplifications may be made in the computational process. Forexample, the volume of the dead volume, standard temperature, andstandard pressure may all be combined into a single constant for usewith the inlet flow measurement and time rate of change of pressurewithin the dead volume to compute a compensated outlet flowapproximation. This simplification would yield an equation of the form:

Qo=Qi−C1(V/T)(dP/dt)  (3)

[0069] where:

[0070] Qo=the compensated sensed inlet flow rate,

[0071] Qi=the sensed inlet flow rate,

[0072] C1=a normalizing constant relating the temperature and pressureto standard temperature and pressure

[0073] V=the volume between the sensor bypass and the outlet flowcontrol valve,

[0074] T=the temperature of the fluid within the volume,

[0075] dP/dt=time rate of change of pressure within the volume.

[0076] As previously noted, the volume V could be folded into theconstant C1. From step 512 the process proceeds to step 514 where itcontinues, with the flow sensor's controller obtaining pressure,temperature, and flow readings and computing a compensated outlet flowestimate, as described. The process proceeds from step 514 to end instep 516, for example, when the mass flow sensor is shut down.

[0077] Returning to the block diagram of FIG. 4, in this illustrativeembodiment, the controller 400 includes a diagnostic interface 422 thatpermits an operator, such as a technician for example, to not onlyinitiate, but conduct diagnostic tests on the mass flow controller.Furthermore, the interface permits the operator to conduct thediagnostics in a manner that requires no input from the local systemcontroller, which may be a workstation, that otherwise normally controlsthe mass flow controller. Such diagnostics are transparent to the localsystem controller, which may not even be made aware of the diagnosticsbeing performed and may, consequently, continue its operations unabated.The diagnostic interface provides access to mass flow controller sensormeasurements, control outputs and mass flow controller diagnostic inputsand outputs. These various inputs and outputs may be exercised andmeasured through the diagnostic interface with very little delay. In anillustrative dual-processor embodiment described in greater detail inthe description related to the discussion of FIG. 9, a deterministicprocessor may modify outputs and/or monitor inputs, from sensors or testpoints, for example. During the execution of on-line diagnostics, thecontroller continues to execute its process control functions,unimpeded, while, at the same time, the controller may provide real-timeinteraction with a technician (i.e., interactions wherein the delays areimperceptible to a human operator) either locally or through atelecommunications connection.

[0078] Using the diagnostic port, an operator can adjust control values,such as the set point, used to determine the mass flow controller'soperation. Additionally, the operator may modify sensor output values inorder to test the mass flow controller's response to specified sensorreadings. That is, an operator can modify the sensor readings a massflow controller employs to control the flow of gasses through its outletvalve and, thereby, exercise the controller for diagnostic purposes. Anoperator may read all sensor and test point inputs as well asinformation stored regarding control (stored by the deterministiccontroller in the dual processor embodiment), read all sensor values,read test point values, read control information, such as the desiredset point. Additionally, the operator may write to control outputs andtest points and over-write stored values, such as sensor readings or setpoint information in order to fully test the controller through thediagnostic port.

[0079] In an illustrative embodiment, a mass flow controller inaccordance with the principles of the present invention may include aweb server. Such a web server may be included within the diagnosticinterface, for example. In such an embodiment, the diagnostic interfaceincludes a web-server that permits the mass flow controller to be usedin a system such as illustrated in the block diagram of FIG. 6. In sucha system, a user, such as a technician, may employ a web-enabled device600 such as a personal computer, personal digital assistant, or cellulartelephone that runs a web browser (e.g., Netscape Explorer) tocommunicate with a server 602 embedded in the mass flow controller 604.The server 602 includes web pages that provide an interface for the userto the mass flow controller 604 in accordance with the principles of thepresent invention. The discussion related to FIGS. 13A through 13Eprovide greater detail related to the web server capability embedded inan illustrative embodiment of a mass flow controller in accordance withthe principles of the present invention.

[0080] Mass flow sensors are typically calibrated during theirmanufacturing process. Because a mass flow sensor is usuallyincorporated into a mass flow controller, this discussion will center onmass flow controllers, but the methods and apparatus discussed hereinare applicable to “standalone” mass flow sensors as well. Thecalibration process requires a technician to supply a gas at a knownflow rate to the mass flow controller and correlate the mass flowsensor's flow signal to the known flow rate. For example, in the case ofa mass flow sensor that provides a voltage output corresponding to flow,the technician maps the voltage output from the sensor into the actualflow rate. This process may be repeated for a plurality of flows inorder to develop a set of voltage/flow correlations: for example, a 4Volt output indicates a 40 standard cubic centimeter per minute (sccm)flow, a 5 Volt output indicates a 50 sccm flow, etc.

[0081] Flow rates that fall between calibration points may beinterpolated using linear or polynomial interpretation techniques, forexample. This process may be repeated for several gases. Correlationtables that relate the signal from the mass flow sensor (which may be avoltage) to flow rates for various gases may thus be developed andstored. Such tables may be downloaded to a mass flow controller for use“in the field”, or may be stored within a mass flow controller. Often,technicians calibrate a mass flow controller using a relativelyinnocuous gas, such as N2, and provide calibration coefficients that maybe used to correlate the flow of another gas to the calibration gas.These calibration coefficients may then be used in the field when aknown gas is “flowed” through the mass flow controller to compute theactual flow from the apparent flow. That is, the apparent flow may be aflow correlated to N2 and, if Arsine gas is sent through the mass flowcontroller, the mass flow controller multiplies the apparent flow by anArsine gas calibration coefficient to obtain the actual flow.Additionally, once in the field, mass flow controllers may bere-calibrated on a regular basis to accommodate “drift”, orientation,water content of a gas the flow of which is being controlled, or tocompensate for other factors. U.S. Pat. No. 6,332,348 B1, issued on Dec.25, 2001 to Yelverton et al., which is hereby incorporated by reference,discusses these factors, and the unwieldy processes and equipmentrequired to carry out these in-the-field calibrations in greater detail.

[0082] A calibration method and apparatus in accordance with theprinciples of the present invention will be described in the discussionrelated to the conceptual block diagram of FIG. 7. This calibrationsystem and method may be employed in a manufacturing setting, or, in anillustrative embodiment, may be incorporated into a self-calibratingmass flow controller. The mass flow controller 700 includes a mass flowsensor 702 and an electronic controller 704 that receives a flow signalfrom the mass flow sensor 702. A calibrator includes a variable flow gassource 708, a receptacle of predetermined volume 710, and a pressuredifferentiator 712. It should be noted that the lines separatingdifferent functional blocks are somewhat fluid. That is, in differentembodiments, the function associated with one block may be subsumed byone or more other blocks. For example, in an illustrative embodiment,the pressure differentiator 712 is implemented all, or in part, by theexecution of code within the electronic controller 704. The variableflow gas source provides a gas at proportional rates to both thereceptacle of predetermined volume and the mass flow sensor. The flowrate to the mass flow sensor 702 may be equal to the flow rate to thereceptacle of predetermined volume 710: i.e., a proportionality constantof 1, for example. The mass flow sensor 702 is configured to produce amass flow signal indicative of the flow that it senses and, in thisillustrative embodiment, this signal is sent to the electroniccontroller 704. The pressure differentiator 712 produces a signalcorrelated to the flow from the variable flow source 708 into thereceptacle of predetermined volume 710 according to the relationship ofequation 4:

Qo=Qi−C1(V/T)(dP/dt)  (4)

[0083] where:

[0084] Qo=the outlet flow rate in standard cubic centimeters per minute,

[0085] Qi=the inlet flow rate in standard cubic centimeters per minute,

[0086] C1=a normalizing constant relating the temperature and pressureto standard temperature and pressure

[0087] V=the predetermined volume of the receptacle in liters,

[0088] T=the Kelvin temperature of the fluid within the receptacle,

[0089] dP/dt=time rate of change of pressure within the receptacle inTorr/second.

[0090] In an illustrative embodiment, the receptacle is closed and gasflows into the receptacle until the pressure within the receptacleequals that of gas supplied by the variable flow source 708. In such anillustrative embodiment, the variable flow source may be aconstant-pressure source that, as pressure within the receptacle builds,supplies gas at an exponentially decreasing flow rate. In such a case,the outlet flow Qo=0, and the inlet flow, Qi is given by:

Qi=C1(V/T)(dP/dt)  (5)

[0091] The pressure differentiator 712 takes the time derivative of thepressure within the receptacle 710 and, given the normalizing constantC1, the predetermined volume V, and the gas temperature within thereceptacle, the differentiator (and/or the electronic controller 704)may determine the actual flow into the receptacle 710. Because the flowinto the receptacle is proportional to the flow into the thermal sensor,the actual flow into the thermal sensor 702 may also be determined by amultiplying the actual flow into the receptacle by a proportionalityconstant (e.g., the proportionality constant is 1 if the flows areequal). The signal from the mass flow sensor is then correlated, by theelectronic controller 704 for example, to the actual flow, determined asjust described. Such correlation relates one or more signal levels fromthe mass flow sensor to the actual flows. The differentiator may includeanalog differentiator circuitry, for example, that takes the timederivative of the pressure signal. The differentiator output signal, asignal representative of the time derivative of the pressure within thereceptacle, may be sampled by an analog-to-digital converter to permitthe electronic controller 704, which may include a microprocessor, DSPchip, or dual processors, for example to operate on the time derivativesignal. Alternatively, the pressure differentiator 712 may convert thepressure signal to digital form for processing by the electroniccontroller 704, which takes the time derivative of the pressure signal.In such an embodiment, the electronic controller, in combination withdifferentiator code, operates as the differentiator. The controlleremploys at least two pressure differences divided by corresponding timeintervals to compute the derivative. The gas may be supplied in parallelto the receptacle and mass flow sensor, or it may be supplied in series,as will be described in greater detail in the following discussionrelated to a self-calibrating mass flow controller.

[0092] In operation, a mass flow controller may be calibrated as justdescribed, using a plurality of gases, with the correlation values(mappings of sensor output to actual flow) stored in tables. Calibrationcoefficients, relating flow measurements of one gas to another may alsobe developed and stored. The tables and/or coefficients may bedownloaded to a mass flow controller in the field for use by thecontroller in controlling the flow of a gas. Various known interpolationtechniques, such as linear or polynomial interpolation may be employedin conjunction with the calibration tables and/or coefficients.Additionally, such stored calibration tables and/or coefficients may beused as default values in a self-calibrating mass flow controller inaccordance with the principles of the present invention. Aself-calibrating mass flow controller in accordance with the principlesof the present invention includes a calibrator 706 and a mass flowsensor 702 which may be employed to calibrate the mass flow controllerin a manner as just described. In the case of a self-calibrating massflow controller, though, the calibration can be performed, In Situ, inthe field just as readily as in a manufacturing setting.

[0093] Once installed in the field, on a semiconductor processing toolas in the system 100 of FIG. 1, for example, the mass flow controllercan calibrate itself using the gas that is to be used during thesemiconductor processing. By using the gas that is to be used inprocessing, the mass flow controller may provide a more accurate flowmeasurement, because it will automatically accommodate variations, suchas moisture content, for example. Additionally, a new processing gas maybe used just as readily as a conventional gas, since theself-calibrating mass flow controller may calibrate itself (that is,correlate mass flow signal levels to actual flow levels determined bythe pressure differentiator), on the gas to be used, not in relation toanother, standard gas, such as N2. Because the mass flow controller iscalibrated in the orientation in which it will be used, discrepanciesdue to re-orientation of the mass flow controller in the field relativeto the position in which it was calibrated during manufacturing will besubstantially eliminated. All the mass flow controllers within a systemsuch as system 100 of FIG. 1 may be calibrated automatically andsimultaneously, within moments. This is in contrast to the cumbersome,painstaking process employed for conventional mass flow controllers,which are typically individually calibrated by a technician employingmultiple mass flow meters, going from mass flow controller to mass flowcontroller. As will be described in greater detail in the descriptionrelated to the discussion of FIG. 8, a mass flow controller thatincludes a thermal mass flow sensor and a pressure transducer may shutits outlet valve to create a varying gas flow into it's dead volume. Bytaking the time derivative of the pressure the actual flow into the deadvolume receptacle may be determined. The mass flow controller'scorrelation of the actual value of the flow to the thermal mass flowsensor signal acts as the mass flow controller's calibration.

[0094]FIG. 8 is a conceptual block diagram of a self-calibrating massflow controller 800 in accordance with the principles of the presentinvention. In this illustrative, series-flow, embodiment a gas flowsthrough a thermal sensor 802 into a receptacle of predetermined volume804, then through an outlet valve 806. The outlet flow Qo would normallybe a controlled flow into a chamber, such as a chamber within anintegrated circuit processing tool. An electronic controller 808, which,in this illustrative embodiment, executes code to perform thedifferentiation required to obtain actual flow, as described in thediscussion related to FIG. 7, is in communication with the thermalsensor 802, pressure sensor of 804 and the outlet valve 806. In anillustrative process, the electronic controller 808 operates inconjunction with the outlet valve 806 to form a variable-flow gassupply. That is, the electronic controller shuts the outlet valve, whichcauses the flow to decrease exponentially. The pressure within the deadvolume increases, and the electronic controller differentiates thissignal a number of times in order to obtain actual flow readings tocorrelate to the mass flow sensor signal values over a relatively broadrange of flows. Additionally, in order to extend the period of timeduring which the flow is varying and to obtain actual flow values forcorrelation with the thermal mass flow signal values over a broad range,the electronic controller may open the outlet valve to a fully openposition before closing it.

[0095] The pressure and flow profiles associated with such a process areillustrated conceptually in the graph of FIG. 9. At an initial time t0the pressure difference between gas at the inlet to the mass flowcontroller Pin and the pressure Pr downstream in the receptacle 804forces gas to flow through the mass flow controller at a rate Qin. Inthis example, the inlet pressure, pressure within the receptacle, andflow through the mass flow controller are constant. At time tso thecontroller shuts the outlet valve, thereby reducing outlet flow Qo tozero. Gas continues to flow into the receptacle as long as there is apressure difference between the receptacle and the inlet. As thepressure within the receptacle rises exponentially toward an equilibriumstate of equality with the inlet pressure, the inlet flow decreases. Bytaking the derivative of the pressure change within the receptacle (alsoreferred to herein as “dead volume” in association with an illustrativeembodiment of the invention), the electronic controller may determinethe actual flow into the receptacle, as previously described.

[0096] The electronic controller may correlate a plurality ofsimultaneous readings produced by the thermal mass flow sensor, tothereby calibrate the mass flow sensor. That is, once this process iscompleted for a specific gas, flow readings from the thermal mass flowsensor may be correlated to actual flow rates. The results may beemployed by the electronic controller 808 to control the opening of thevalve 806 in a closed loop control system in order to deliver a selectedflow downstream. In order to extend the period from the time thecontroller shuts the valve, tso, and the time at which the flow becomesundetectable, and to thereby increase the number and precision ofpressure measurements that may be made, the controller may open thevalve completely before shutting it at time t0. Additionally, one ormore flow restrictors may be placed in the flow path between the inletto the thermal mass flow sensor and the inlet to the receptacle 804.

[0097] The conceptual block diagram of FIG. 10 illustrates thearchitecture of a dual-processor embodiment of an electronic controller1000 such as may be used in a mass flow sensor in accordance with theprinciples of the present invention. In this illustrative embodiment,the controller includes two processors 1002, 1004. One of the processors1002 is dedicated to “real time” processes and the other processor 1004is dedicated to non-real time processes. By “real time” we meanprocesses that require a specified level of service within a boundedresponse time. In this sense, the processes are deterministic and theprocessor 1002 will be referred to herein as the deterministicprocessor. The objective of the dual processor architecture is to reducethe number of interrupts and manage asynchronous event responses in apredictable way. The non-deterministic processor 1004 may handleevent-driven interrupts, such as responding to input from a user. Thedeterministic processor 1002 handles only frame-driven, that is,regularly scheduled, interrupts. In an illustrative embodiment, thenon-deterministic processor is a general purpose processor, suited for avariety of tasks, such as user-interface, and other, miscellaneoustasks, rather than a specialized co-processor, such as a math- orcommunications-coprocessor. In particular, a TMS320VC5471, availablefrom Texas Instruments, Inc., may be employed in a dual-processorembodiment in accordance with the principles. The TMS320VC5471 isdescribed in a data manual, available athttp://www-s.ti.com/sc/ds/tms320vc5471.pdf, which is hereby incorporatedby reference.

[0098] A processor interface 1006 provides for inter-processorcommunications. The deterministic processor 1002, includes sensor andactuator interfaces. Among the sensor interfaces, a flow sensorinterface 1008 operates in conjunction with a mass flow sensor toproduce a digital representation of the rate of mass flow in anassociated mass flow controller. One or more actuator interfaces 1010are employed by the deterministic processor 1002 to control the openingof an associated mass flow controller's output control valve or drive adiagnostic test point, for example. The actuator may be a current-drivensolenoid or a voltage-driven piezo-electric actuator, for example. Aswill be described in greater detail in the discussion related to theflow chart of FIG. 9, after initialization, the deterministic processor1002 loops through a control sequence, gathering sensor data, gatheringsetting information (for example, a desired mass flow setting),providing status information, and controlling the state of the outletvalve. Because non-deterministic tasks are offloaded to thenon-deterministic processor 1004, the deterministic processor's controlloop may be very compact. Consequently, control tasks may be executedwithin a minimal period of time and control readings and drive signalsmay be updated more frequently than possible if time were set aside forservicing non-deterministic tasks.

[0099] The controller 1000 operates in conjunction with a thermal massflow sensor as generally described in the discussion related to FIG. 3to produce a digital representation of the rate of mass flow into anassociated mass flow controller. The digital representation may take theform of one or more data values and is subject to fluctuations due topressure transients at the input of the mass flow sensor. The controller1000, and more specifically, the deterministic processor 1002 may employdata obtained at the pressure sensor interface 1006 to compensate forfluctuations induced in the thermal mass flow sensor by pressuretransients on the mass flow sensor inlet line. In this illustrativeembodiment, the deterministic processor 1002 employs the temperature,pressure, and mass flow readings obtained from the respective 1008,1007, and 1005 interfaces, to produce a compensated mass flow readingthat more closely reflects the mass flow at the outlet of the mass flowsensor than a reading from the thermal mass flow sensor alone. Thedeterministic processor 1002 also provides control to sensors, asnecessary, through thermal flow 1005, pressure 1007, and temperature1008 sensor interfaces. The compensation process will be described ingreater detail in the discussion related to FIG. 11. The deterministicprocessor 1002 also includes a valve actuator interface 1010, which thedeterministic processor employs to control the position of a valve, suchas the valve 220 of FIG. 2, to thereby control the rate of fluid flowthrough a mass flow controller, such as the mass flow controller 200, ina closed-loop control process.

[0100] The deterministic processor 1002 is devoted to the closed-loopvalve control process, and, consequently, must be capable of operatingwith sufficient speed to read the various sensor outputs, compensate asnecessary, and adjust the valve to produce a predetermined flow rate.The flow rate is predetermined in the sense that it is “desired” in somesense, and it need not be a static setting. That is, the predeterminedflow rate may be set by an operator using a mechanical means, such as adial setting, or may be downloaded from another controller, such as aworkstation, for example, and updated frequently. Typically, gas flowcontrol, and in this case, compensated gas flow control, requiresrelatively high-speed operation. Various types of processors, such asreduced instruction set (RISC), math coprocessor, or digital signalprocessors (DSPs) may be suitable for such high-speed operation. Thecomputational, signal conditioning, and interfacing capabilities of aDSP make it particularly suitable for operation as the deterministicprocessor 1002. As will be described in greater detail in thedescription of the control process related to the discussion of FIG. 9,the function performed by the deterministic processor 1002 isdeterministic in the sense that certain operations are completed in atimely and regular manner in order to avoid errors, and possibleinstabilities, in the control process. The deterministic 1002 andnon-deterministic 1004 processors communicate via the inter-processorinterface 1006 in a manner that does not impede the deterministicoperation of the deterministic processor 1002. Inter-processorcommunications are discussed in greater detail in the discussion relatedto FIG. 9.

[0101] The non-deterministic processor 1004 includes a local userinterface 1016 that may be used with one or more input devices, such asa keypad, keyboard, mouse, trackball, joy stick, buttons, touch screens,dual inline packaged (DIP) or thumb-wheel switches, for example, toaccept input from users, such as technicians who operate a mass flowcontroller associated with the non-deterministic processor 1004. Thelocal user interface 1016 also includes one or more outputs suitable fordriving one or more devices, such as a display, which may be acharacter, alphanumeric, or graphic display, for example, indicatorlight, or audio output device used to communicate information from amass flow controller to a user. A communications interface 1018 permitsa mass flow controller to communicate with one or more otherinstruments, and/or with a local controller, such as a workstation thatcontrols a tool that employs a plurality of mass flow controllers and/orother devices in the production of integrated circuits, for example. Inthis illustrative example, the communications interface 1018 includes aDeviceNet interface. A diagnostic interface 1020 provides an interfacefor a technician to run diagnostics, as previously described in relationto the diagnostic interface 422 of FIG. 4. In an illustrativeembodiment, the diagnostic interface includes an Ethernet interface anda web server.

[0102] The compactness of code for the deterministic processor 1002permits the deterministic processor to be highly responsive to inputchanges and to quickly modify actuator signals in response to thosechanges. This partitioning of operations between deterministic andnon-deterministic processors also eases the initial development of code,for both the deterministic and non-deterministic processors. Forexample, the deterministic code needn't respond to unscheduled events,such a “mirroring” a user's requests on a display at a user interface,and the non-deterministic code needn't break away from providing suchuser feedback in order to adjust an outlet valve control setting everyfifty bus cycles. The partitioning between deterministic andnon-deterministic also permits relatively simple revisions and upgrades.If the code for one processor must be revised or upgraded, the code forthe other may require no revisions or only minor revisions. Inparticular, the code for the deterministic processor may be more“mature”, or fixed than that for the non-deterministic processor; userinterfaces, communications and other similar functions tend to beupgraded more frequently than the deterministic, mass flow control,functions.

[0103] Using this illustrative dual-processor embodiment, a userinterface may be updated without any impact on the control functioncode, for example. Revision and maintenance of mixed-mode code(deterministic and non-deterministic code) would be a much morecomplicated and costly proposition than code partitioned in a manner inaccordance with the principles of the present invention. In anillustrative embodiment the dual-processor controller 1000 may by ahybrid processor that incorporates two processors on one integratescircuit. An integrated circuit such as the TMS320C5471 hybrid processoravailable from Texas Instruments may be employed as the dual processorsin accordance with the principles of the present invention. The digitalsignal processing (DSP) subsystem of the chip, due to its mathcapabilities would be more suitable as the deterministic processor insuch an application. The IC's dual-ported memory may be employed as theinter-processor interface, with the processors writing to and readingfrom memory locations set aside to act as “mail boxes” for the transferof information, including data, commands, and command responses.

[0104] Such an inter-processor interface permits the deterministicprocessor to continue operating in a frame-driven mode while, at thesame time, allowing the deterministic processor to play a role indiagnostics and calibration. Any request for sensor data from thenon-deterministic processor may be picked up from the mailbox on onepass of the deterministic processor's control loop, with the readingsdeposited in the mailbox the very next time through the loop. Diagnosticoutputs may be modified similarly. The deterministic processor may alsooperate in other, non-process oriented modes. For example, during aself-calibration process such as previously described, the deterministicprocessor would no longer operate to maintain a set flow through themass flow controller. In such a mode the deterministic processor wouldbe occupied by shutting the mass flow controller's outlet valve, takinga plurality of time derivatives of the pressure within the dead volume,computing the corresponding actual flow in the mass flow controller, andcorrelating the actual flow to the flow signal produced by a thermalmass flow sensor.

[0105] The flow chart of FIG. 11 outlines the process of sensing andcontrolling the flow of gas through a dual-processor mass flowcontroller in accordance with the principles of the present invention.The process begins in step 1100 and proceeds from there to step 1102where the controller is initialized. This initialization step mayinclude the uploading of calibration values or a calibration sequenceitself. Additionally, operating code for both the deterministic andnon-deterministic processors may be uploaded at this point. In anillustrative embodiment, the non-deterministic processor may upload itsown code and begin operating, then upload code for the deterministicprocessor. In the process of uploading code for the deterministicprocessor, the non-deterministic processor may select among a pluralityof executable code sets to upload to the deterministic processor,thereby tailoring the operation of the deterministic processor. Thenon-deterministic processor may base this selection on switch settings,commands from a local controller (e.g., a workstation controlling theoperation of a semiconductor process tool), or settings stored innon-volatile storage, for example. Such a selection permits a mass flowcontroller to be tailored to different flow control operations. Forexample, a technician may, by selecting among code sets, choose tooperate the controller in a “pressure controller” mode rather than a“mass flow controller” mode, and this selection may be made locally orremotely (i.e., through a telecommunications link).

[0106] In step 1104 the non-deterministic processor passes operatingcode and initial control settings to the deterministic processor whichthen begins operating in a manner described generally in connection withthe flow chart of FIG. 12. From step 1104, the process proceeds to step1106 where the non-deterministic processor services the local input/outinterface. Such servicing may include reading various inputs, includingkeyboard, switch, or mouse inputs, and displaying information locally,through LEDS, alphanumeric displays, or graphical displays. From step1106 the process proceeds to step 1108 where the non-deterministicprocessor services the communications interface. This servicing mayinclude the steps of uploading control and sensor data to a workstationthat operates as the local controller of a semiconductor process tool,for example. Additionally, the non-deterministic processor may downloadupdated settings from the local controller.

[0107] From step 1108 the process proceeds to step 1110 where thenon-deterministic processor services the diagnostic interface. Variousdiagnostic operations, such as set forth in the description related tothe discussion of FIG. 4, may be performed in this step. In anillustrative embodiment, the mass flow controller includes a web server,which permits an operator to run diagnostics through a network such asthe “world wide web.” From step 1110 the process proceeds to step 1112where the non-deterministic processor services the inter-processorinterface. During “normal”, non-diagnostic operation, thenon-deterministic processor obtains readings from the deterministicprocessor and passes control information, such as a flow settingobtained through the communications interface, to the deterministicprocessor. From step 1112, the process proceeds to continue theprocesses just set forth in step 1114. The process proceeds to end instep 1116 when the mass flow controller is turned off, for example.

[0108] As previously noted, the steps set forth in this and other flowcharts herein need not be sequential and, in fact, a number of functionsperformed by the non-deterministic processor may beevent-interrupt-driven and no predictable sequence may be ascribed tothe non-deterministic processor's operation. Other processes, such asdata-logging may be performed at regular intervals. Thenon-deterministic processor can support a two-way socket connection tothe deterministic processor through an Ethernet network interface, forexample, to provide a relatively direct connection between a remote userand the deterministic processor.

[0109] The flow chart of FIG. 12 depicts the operation of thedeterministic processor of a dual processor mass flow controller inaccordance with the principles of the present invention. In the contextof this flow chart, is assumed that an initialization process has takenplace and that the deterministic processor is cycling through itscontrol loop. The process begins in step 1200 and proceeds from there tostep 1202 where the deterministic processor determines whether it is tooperate in its “normal” control capacity or whether it is to operate inan alternative mode, such as a manual diagnostic mode or an automaticdiagnostic mode, for example. The deterministic processor bases thisdecision on information it obtains from the inter-processor interface.The deterministic processor services frame-driven, rather thanevent-driven interrupts; consequently, it regularly polls theinter-processor interface to obtain information such as this.

[0110] If the deterministic processor is to operate in its normal mode,the process proceeds from step 1202 to step 1204, where thedeterministic processor obtains information from the inter-processorinterface regarding the desired control settings. This information maybe in the form of a desired flow rate received from a local controller,from a front panel user interface, or through the diagnostic port forexample. The deterministic processor may also transfer information, suchas sensor data, for example, to the non-deterministic processor throughthe inter-processor interface during this step. From step 1204 theprocess proceeds to step 1206 where the deterministic processor gathersdata, from a variety or sensors for example. The sensors from which thedeterministic processor obtains data may include a mass flow sensor(thermal or other type), a temperature sensor, or a pressure sensor, forexample.

[0111] From step 1206 the process proceeds to step 1208, where thedeterministic processor computes the flow rate of material through themass flow controller. In an illustrative embodiment, the mass flowcontroller includes a thermal mass flow sensor and a pressure sensorconfigured to measure the pressure within the dead volume between thethermal mass flow sensor's bypass and the mass flow controller outletvalve. In this embodiment, the deterministic processor may employ themethod described in relation to the discussion of FIG. 5 to compensate aflow rate measured by a thermal mass flow sensor at the inlet of thecontroller to more closely approximate the flow rate at the outlet ofthe controller. In an embodiment in which the flow rate obtained fromthe sensor is not compensated, the process would proceed directly fromstep 1206 to step 1210, skipping the computational process of step 1208.

[0112] In step 1210 the deterministic processor determines whether theflow rate computed in step 1208 (or read in step 1206) is equal to thedesired flow rate indicated by the setting information obtained from thenon-deterministic processor via the inter-processor interface in step1204. If the values are equal the deterministic processor continues theoperation as just described, as indicated by the “continue” block 1214(i.e., the deterministic processor returns to step 1202 and continues tocycle through the loop). If the values are not equal, the deterministicprocessor computes an error signal and employs the error signal toadjust the drive signal to the mass flow controller's outlet valve. Fromstep 1212 the process proceeds to continue in step 1214. The processproceeds from step 1214 to end in step 1216 when the mass flowcontroller is shut down or reset, for example.

[0113] If, in step 1202 the deterministic processor concludes that it isnot to operate in the normal mode, the process proceeds throughconnecting box A to step 1218, where the deterministic processordetermines whether it is to operate in a diagnostic mode. Thedeterministic processor may obtain this information from theinter-processor interface. If the deterministic processor is to operatein a diagnostic mode, the process proceeds to step 1220. In step 1220the deterministic processor determines which diagnostic mode it is tooperate in. Once again, this information may be passed to thedeterministic processor through the inter-processor interface. In an“automatic” mode, the deterministic processor acquires a sequence ofdiagnostic values from the inter-processor interface. The sequence ofvalues is available at the interface for acquisition by thedeterministic processor. The diagnostic values may be control outputs,for setting the opening of the mass flow controller outlet valve or forsetting test point drive values, for example, or the diagnostic valuesmay indicate desired sensor readings or readings from test points, forexample. The diagnostic values may also indicate the sequence in whichthe values are to be employed, in order to set test point driver values,then read test point outputs, for example. In a manual mode, diagnosticvalues are made available to the deterministic processor through theinter-processor interface one at a time. In an embodiment in which themass flow controller includes a web server, a technician may use aweb-enabled workstation to contact the server in the mass flowcontroller. Once linked to the server, the technician may enter a valvesetting command, by typing, selecting from a pull down menu or clickingon icon, for example. This single, setting, command would be received bythe non-deterministic processor through its diagnostic port and passedto the deterministic processor through the inter-processor interface.

[0114] In the manual diagnostic mode the deterministic processorexecutes through whatever diagnostic values are available at theinter-processor interface, then returns to it's normal control loop.This could “override” a single control loop cycle if, for example, asingle diagnostic value, such as a test point drive value, is presentedto the deterministic processor or, if a sequence of diagnostic values ispresented to the deterministic processor, a number of control loopcycles may be overridden. In the automatic diagnostic mode a number ofdiagnostic values may be exchanged through the inter-processor interfacein a period corresponding to a few control loop cycles, with asubstantial number, on the order of at lest ten times as many, controlloop cycles intervening between automatic diagnostic exchanges.Diagnostic modes may be combined, for example, to produce an automaticactive on-line diagnostic mode, for example. In an illustrativeembodiment, a mass flow controller in accordance with the principles ofthe present invention operates on a one-millisecond control loop cycle,during which it provides one percent of full-scale accuracy.

[0115] Keeping the various diagnostic modes in mind, and keeping in mindthat processes illustrated through the use of flow charts may not bestrictly linear processes and alternative flows may be implementedwithin the scope of the invention, the diagnostic process will bedescribed generally in relation to steps 1220 through 1226. In step 1220the deterministic processor acquires diagnostic values from theinter-processor interface. As previously noted, these values may be forthe deterministic processor to use as control outputs or they mayindicate data that is to be acquired by the deterministic processor,from a sensor, for example. From step 1220 the process proceeds to step1222 where the deterministic processor processes the values acquired instep 1220, by changing an outlet valve actuator drive signal ortransferring a sensor reading to the inter-processor interface, forexample.

[0116] From step 1222 the process proceeds to step 1224 where thedeterministic processor determines whether it has completed itsdiagnostic tasks. If it has not completed its diagnostic tasks, forexample if it is operating in the automatic diagnostic mode and thereare more values in a sequence of values to be retrieved from theinter-processor interface, the process returns to step 1222 and fromthere as previously described. If, in step 1226 the deterministicprocessor concludes that it has completed its diagnostic task, theprocess returns through connecting box B to step 1214 of FIG. 11. If thedeterministic processor determines that it is not to operate in adiagnostic mode, the process proceeds from step 1218 where processorperforms functions such as routine background operations, then proceedsto return through connecting block B to step 1214 and from there aspreviously described.

[0117] The screen shots of FIGS. 11A through 11E illustrate a userinterface such as may be made available for access to a mass flowcontroller in accordance with the principles of the present inventionthat includes a web server interface, such as the interface 608 of FIG.6. In an illustrative embodiment the mass flow controller includes a webserver, such as the server 602 of FIG. 6. A user may employ the serverlocally, through a local controller, or remotely, from a web-enableddevice, such as the device 610 of FIG. 6. In this manner, the same userinterface may be employed for both remote and local interactions withthe mass flow controller. Detailed information regarding a mass flowcontroller, such as model number, range, and manufacturing setupparameters, may be displayed to a user and user-changeable setupparameters may be displayed as well. Different display techniques may beemployed. If there are only a limited number of acceptable values, theymay be displayed and chosen from a pulldown menu, for example. Aspreviously described, a user, such as a technician can change set pointvalues, open or close a valve, or monitor flow output, for example,through this interface. Additionally, while the mass flow controller isoperating under a process control application, a user may induce theserver to plot and log parameter values obtained from the mass flowcontroller.

[0118] The screen shot of FIG. 13A illustrates the display a user mayencounter when first accessing a mass flow controller in accordance withthe principles of the present invention over the web. The displayprompts the user to choose a communications protocol through use of thepulldown window 1300. The “query devices” link 1302 allows the user toinitiate a process whereby his browser attempts to locate all devicesthat it recognizes.

[0119] Basic information may be downloaded through the server.Information related to the mass flow controller are displayed in thescreen of FIG. 13B. Such screens may be expanded or collapsed. A usermay choose to view information related to a subset of the displayed massflow controllers. Based on the model number, serial number andinternally stored codes, product specifications for the mass flowcontroller are displayed along with user-selectable parameters, whichmay be displayed in a list, for example. A user may employ this screento download calibration data to or from a mass flow controller and toenter calibration tables. A user may also alter set points through thisinterface and monitor the reported flow through the corresponding massflow controller. Additionally, a user may override settings and open orclose a mass flow controller's outlet control valve. Each mass flowcontroller's specifications may be viewed, as illustrated by the screenof FIG. 13C. Illustrative user-selectable parameters are displayed inthe screen shot of FIG. 13D and calibration data such as a user maydownload from a mass flow controller is illustrated in the screen shotof FIG. 13E.

[0120] A software implementation of the above described embodiment(s)may comprise a series of computer instructions either fixed on atangible medium, such as a computer readable media, e.g. diskette,CD-ROM, ROM, or fixed disc, or transmittable to a computer system, via amodem or other interface device, such as communications adapterconnected to the network over a medium. Medium can be either a tangiblemedium, including but not limited to, optical or analog communicationslines, or may be implemented with wireless techniques, including but notlimited to microwave, infrared or other transmission techniques. Theseries of computer instructions embodies all or part of thefunctionality previously described herein with respect to the invention.Those skilled in the art will appreciate that such computer instructionscan be written in a number of programming languages for use with manycomputer architectures or operating systems. Further, such instructionsmay be stored using any memory technology, present or future, including,but not limited to, semiconductor, magnetic, optical or other memorydevices, or transmitted using any communications technology, present orfuture, including but not limited to optical, infrared, microwave, orother transmission technologies. It is contemplated that such a computerprogram product may be distributed as a removable media withaccompanying printed or electronic documentation, e.g., shrink wrappedsoftware, preloaded with a computer system, e.g., on system ROM or fixeddisc, or distributed from a server or electronic bulletin board over anetwork, e.g., the Internet or World Wide Web.

[0121] Although various exemplary embodiments of the invention have beendisclosed, it will be apparent to those skilled in the art that variouschanges and modifications can be made which will achieve some of theadvantages of the invention without departing from the spirit and scopeof the invention. It will be apparent to those reasonably skilled in theart that other components performing the same functions may be suitablysubstituted. Further, the methods of the invention may be achieved ineither all software implementations, using the appropriate object orprocessor instructions, or in hybrid implementations that utilize acombination of hardware logic, software logic and/or firmware to achievethe same results. Processes illustrated through the use of flow chartsmay not be strictly linear processes and alternative flows may beimplemented within the scope of the invention. The specificconfiguration of logic and/or instructions utilized to achieve aparticular function, as well as other modifications to the inventiveconcept are intended to be covered by the appended claims.

[0122] The foregoing description of specific embodiments of theinvention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and many modifications andvariations are possible in light of the above teachings. The embodimentswere chosen and described to best explain the principles of theinvention and its practical application, and to thereby enable othersskilled in the art to best utilize the invention. It is intended thatthe scope of the invention be limited only by the claims appendedhereto.

What is claimed is:
 1. A self-calibrating mass flow controller,comprising: a mass flow sensor configured to produce an electronicsignal representative of mass flow in the mass flow controller as afunction of the flow sensed by the sensor; a mass flow calibratoroperative to produce an independent electronic signal representative ofmass flow in the mass flow controller as a function of the flow sensedby the calibrator; and an electronic controller configured to correlatethe mass flow signal from the thermal mass flow sensor to the rate offlow indicated by the independent electronic signal from the mass flowcalibrator.
 2. The mass flow controller of claim 1 further comprising: avalve operative to control the flow of gas in the mass flow controllerunder control of the electronic controller.
 3. The self-calibrating massflow controller of claim 1 wherein the mass flow sensor is a thermalmass flow sensor.
 4. The self-calibrating mass flow controller of claim1 wherein the electronic controller is configured to employ thecorrelated mass flow sensor signal to control the outlet valve duringnon-calibration operation.
 5. The self-calibrating mass flow controllerof claim 1 wherein the electronic controller is configured to store thecorrelation between the mass flow sensor signal and the mass flowcalibrator signal.
 6. A mass flow controller as in claim 1 wherein themass flow calibrator, comprises: a variable flow gas source; areceptacle of predetermined volume configured to receive gas from thevariable flow gas source, the variable flow gas source configured toprovide proportionate flow to the mass flow sensor and to thereceptacle; and a pressure differentiator configured to produce anelectronic signal representative of the time derivative of gas pressurewithin the receptacle of predetermined volume, said time derivativesignal being proportional to the mass flow signal of the mass flowcalibrator.
 7. The mass flow controller of claim 6 wherein the gas flowsource is configured to supply the same gas at the same flow rate toboth the mass flow sensor and the receptacle of predetermined volume. 8.The mass flow controller of claim 6 wherein the gas flow source isconfigured to supply gas to the mass flow sensor and to the receptacleof predetermined volume in series.
 9. The mass flow controller of claim6 wherein the differentiator includes: a pressure transducer configuredto produce an electronic signal representative of the pressure withinthe receptacle; analog differentiator circuitry configured to produce anelectronic signal that is representative of the time derivative of saidelectronic signal representative of the pressure within the receptacle;and an analog to digital converter configured to convert one or morevalues of the analog time derivative signal to digital samples of thetime derivative.
 10. The mass flow controller of claim 9 furthercomprising: storage for storing one or more samples of the signalrepresentative of the time derivative of gas pressure.
 11. The mass flowcontroller of claim 1 wherein the electronic controller is configured tocorrelate the mass flow signal from the mass flow sensor to that of themass flow calibrator for a plurality of gas flows and to produce one ormore compensation coefficients.
 12. The mass flow controller of claim 11wherein the mass flow controller is configured to store the one or morecompensation coefficients.
 13. The mass flow controller of claim 12wherein the mass flow controller is configured to compute the mass flowof a gas using one or more of the stored compensation coefficients andthe flow signal from the mass flow sensor.
 14. The mass flow controllerof claim 6 wherein the differentiator includes: a pressure transducerconfigured to produce an electronic signal representative of thepressure within the receptacle; an analog to digital converterconfigured to convert one or more values of the pressure transducersignal to digital form; and a digital differentiator configured toproduce a plurality of digital values representative of the timederivative of the pressure signal.
 15. The mass flow controller of claim14 further comprising: storage for storing one or more samples of thesignal representative of the time derivative of gas pressure.
 16. Themass flow controller of claim 6 wherein the gas source is configured toprovide an exponentially variable flow gas.
 17. The mass flow controllerof claim 16 wherein the gas source is configured to provide anexponentially increasing gas flow.
 18. The mass flow controller of claim15 wherein the gas source is configured to provide an exponentiallydecreasing gas flow.
 19. The mass flow controller of claim 6 wherein thegas source is configured to provide a patterned gas flow, the patternincluding both increasing and decreasing gas flow.
 20. Aself-calibrating mass flow controller comprising: a thermal mass flowsensor configured to sense the flow of gas in the mass flow controllerand to produce an electronic signal representative of the sensed flow;an electronic controller; an outlet valve operated by the controller toallow a predetermined flow of gas through the mass flow controller; anda pressure sensor operative to measure the pressure of gas within thereceptacle formed between the thermal mass flow sensor's bypass, thereceptacle thus-formed being of predetermined volume, the electroniccontroller configured to operate the outlet valve to vary the flowthrough the mass flow controller, to take the time derivative of thepressure signal, to normalize the time derivative signal, and tocorrelate the signal from the thermal mass flow sensor to the normalizedtime derivative signal.
 21. The mass flow controller of claim 20 whereinthe controller is configured to employ analog differentiator circuitryto take the derivative of the pressure signal.
 22. The mass flowcontroller of claim 20 wherein the controller is configured to computeat least two pressure differences and divide the differences bycorresponding time spans to create the derivative of the pressuresignal.
 23. The mass flow controller of claim 20 wherein the electroniccontroller is configured to correlate the mass flow signal from the massflow sensor to that of the normalized time derivative for a plurality ofgas flows and to produce one or more compensation coefficients relatingthe thermal mass flow signal of one gas flow to that of another.
 24. Themass flow controller of claim 23 wherein the mass flow controller isconfigured to store the one or more compensation coefficients.
 25. Themass flow controller of claim 24 wherein the mass flow controller isconfigured to compute the mass flow of a gas using one or more of thestored compensation coefficients and the flow signal from the mass flowsensor.
 26. A method of self-calibrating a mass flow controller thatincludes a mass flow sensor, an electronic controller, and a mass flowcalibrator, comprising the steps of: a mass flow controller's mass flowsensor producing an electronic signal representative of mass flow in themass flow controller as a function of the flow sensed by the sensor; themass flow controller's mass flow calibrator producing an independentelectronic signal representative of mass flow in the mass flowcontroller as a function of the flow sensed by the calibrator; and themass flow controller's electronic controller correlating the mass flowsignal produced by the mass flow sensor in response to its sensed flowwithin the mass flow controller to the flow indicated by the independentelectronic signal produced by the mass flow calibrator in its sensing offlow in the mass flow controller.
 27. The method of claim 26 furthercomprising the step of: the mass flow controller's electronic controllercontrolling a valve to control the flow of gas in the mass flowcontroller.
 28. The method of claim 27 wherein a thermal mass flowsensor produces the mass flow electronic signal.
 29. The method of claim26 wherein the electronic controller employs the correlated mass flowsensor signal to control the outlet valve during non-calibrationoperation.
 30. The method of claim 26 wherein the electronic controlleris configured to store the correlation between the mass flow sensorsignal and the mass flow calibrator signal.
 31. A method as in claim 26wherein the mass flow calibrator provides: a variable flow gas source; areceptacle of predetermined volume that receives gas from the variableflow gas source, the variable flow gas source providing proportionateflow to the mass flow sensor and to the receptacle; and a pressuredifferentiator that produces an electronic signal representative of thetime derivative of gas pressure within the receptacle of predeterminedvolume, said time derivative signal being proportional to theun-normalized mass flow signal of the mass flow calibrator.
 32. Themethod of claim 31 wherein the gas flow source supplies the same gas atthe same flow rate to both the mass flow sensor and the receptacle ofpredetermined volume.
 33. The method of claim 31 wherein the gas flowsource supplies gas to the mass flow sensor and to the receptacle ofpredetermined volume in series.
 34. The method of claim 31 wherein thedifferentiator provides: a pressure transducer that produces anelectronic signal representative of the pressure within the receptacle;analog differentiator circuitry that produces an electronic signal thatis representative of the time derivative of said electronic signalrepresentative of the pressure within the receptacle; and an analog todigital converter that converts one or more values of the analog timederivative signal to digital samples of the time derivative.
 35. Themethod of claim 34 further comprising the step of: storing one or moresamples of the signal representative of the time derivative of gaspressure.
 36. The method of claim 26 wherein the electronic controllercorrelates the mass flow signal from the mass flow sensor to that of themass flow calibrator for a plurality of gas flows and produces one ormore compensation coefficients.
 37. The method of claim 36 wherein themass flow controller stores the one or more compensation coefficients.38. The method of claim 37 wherein the mass flow controller computes themass flow of a gas using one or more of the stored compensationcoefficients and the flow signal from the mass flow sensor.
 39. Themethod of claim 31 wherein the differentiator provides: a pressuretransducer that produces an electronic signal representative of thepressure within the receptacle; an analog to digital converter thatconverts one or more values of the pressure transducer signal to digitalform; and a digital differentiator that produces a plurality of digitalvalues representative of the time derivative of the pressure signal. 40.The method of claim 39 further comprising the step of: storing one ormore samples of the signal representative of the time derivative of gaspressure.
 41. The method of claim 31 wherein the gas source provides anexponentially variable flow gas.
 42. The method of claim 41 wherein thegas source provides an exponentially increasing gas flow.
 43. The methodof claim 40 wherein the gas source provides an exponentially decreasinggas flow.
 44. The method of claim 31 wherein the gas source provides apatterned gas flow, the pattern including both increasing and decreasinggas flow.
 45. A method of calibrating a self-calibrating mass flowcontroller comprising the steps of: a thermal mass flow sensor sensingthe flow of gas in the mass flow controller and producing an electronicsignal representative of the sensed flow; an electronic controlleroperating an outlet valve to allow a varied flow of gas through the massflow controller; and a pressure sensor measuring the pressure of gaswithin the receptacle formed between the thermal mass flow sensor'sbypass, the receptacle thus-formed being of predetermined volume; theelectronic controller taking the time derivative of the pressure signal,normalizing the time derivative signal, and correlating the signal fromthe thermal mass flow sensor to the normalized time derivative signal.46. The method of claim 46 wherein analog differentiator circuitry takesthe derivative of the pressure signal.
 47. The method of claim 45wherein the controller computes at least two pressure differences anddivides the differences by corresponding time spans to create thederivative of the pressure signal.
 48. The method of claim 45 whereinthe electronic controller correlates the mass flow signal from the massflow sensor to that of the normalized time derivative for a plurality ofgas flows; and produces one or more compensation coefficients relatingthe thermal mass flow signal of one gas flow to that of another.
 49. Themethod of claim 48 wherein the mass flow controller stores the one ormore compensation coefficients.
 50. The method of claim 49 wherein themass flow controller computes the mass flow of a gas using one or moreof the stored compensation coefficients in combination with the flowsignal from the mass flow sensor.
 51. The method of claim 45 wherein thethermal mass flow and pressure sensors measure in situ.
 52. The methodof claim 26 wherein the mass flow sensor and calibrator produceindependent electronic signals representative of mass flow in situ. 53.The method of claim 26 wherein gas flow is restricted in order todifferentiate the pressure over a greater period of time than would beavailable with unrestricted flow.
 54. The method of claim 45 wherein gasflow is restricted in order to differentiate the pressure over a greaterperiod of time than would be available with unrestricted flow.